<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text">
<input type="button" value="平方" onclick="f()">
<input type="button" value="求和" onclick="f1()">
<div>结果:</div>
<script>
    let d = document.querySelector("div");
    let i = document.querySelector("input");
    function f1() {
      //当字符串进行-*/运算时会自动将字符串转成数值
      //d.innerText="结果:"+(i.value*1+i.value*1);//+是拼接,*1是转成数值
        d.innerText="结果:"+(parseFloat(i.value)+parseFloat(i.value));
    }
    function f() {

        //判断用户输入的内容是否是数值 如果是NaN代表不是数值
        if(isNaN(i.value)){
            d.innerText="输入错误!";
            return;
        }
        d.innerText ="结果:"+i.value*i.value;
    }

</script>
</body>
</html>